Most creators working in comics today are SELF-EMPLOYED, and NOT employed by the company that publishes their work. Of course there are exceptions to this, but this is true for the vast majority of creators working in comics today. This SELF-EMPLOYMENT has many implications:
Based on discussions with a few people "in-the-know" about the current pay rates, the following summary reasonably represents the pay scales among the industry's bigger publishers.
THIS IS A GUIDE, REALLY, AND NOT MEANT TO BE USED IN ANY NEGOTIATIONS YOU MAY FIND YOURSELF IN!
Artists get paid by the page (in fact, so do most other creators like writers, letterers, colorists). A typical comic book is 22 pages of interior art, and 1 page of cover art. Covers (and pin-up illustrations) usually pay a little better than the interior art pages -- so for this example, let's just estimate that the pay for one cover (or for one pin-up) is 1.5 times the pay for one interior art page. Doing the math yields one full comic at approximately 23.5 page rates (22 interior page rates + 1.5 page rates for the cover).
Here is a GUIDE for PENCILING page rates:
Page rates for inkers are less than those shown above for pencilers (I'm guessing it's maybe 60%-75% of the rates for pencilers). You should also know that the current industry does not support many (if any) contracts to inkers.
Not only do you get paid to create the art, but the art is returned to the creators. As long as the creators do not republish the art, they are free to either keep it or to sell it (again, under the stipulation that it is not do be republished).
Most companies return 2/3 of the art to the penciler, and 1/3 of the art to the inker. The exact method of determining which pages go where can be different, but the balance of pages returned is pretty much the same. Some companies shuffle the art, and select pages at random. Others simply pluck 1/3 off the book, one month from the front, one month from the back, and so on. Covers are returned similarly: usually, from 3 consecutive covers, 2 consecutive covers are returned to the penciler, and the 3rd goes back to the inker.